The present invention relates generally to tapped delay lines and more specifically to those devices utilizing surface acoustic waves coupled from one piezoelectric substrate to a bulk wave in an adjacent piezoelectric semiconductor substrate by means of piezoelectric coupling across an air gap.
Previously, SAW delay lines could be tapped by the same means that one uses to generate and detect SAWs. These techniques have included interdigital transducers, and other variations which are reviewed in an article entitled "Surface Elastic Waves" by R. M. White, appearing in the Proceedings of the IEEE, Vol. 58, No. 8, August 1970. There are several disadvantages associated with these techniques; they can be lossy, require intimacy with the substrate (e.g. metal interdigital transducers), introduce reflections, and mass load. Also, the tapped energy will always have a delay time shorter than the normal delay between the input and output unless the tapped energy is transferred to another delay line requiring more electrical connections. Applicant's device provides a method for extracting energy from the SAW delay line, delaying it longer than the normal time, without introducing large insertion losses, and without requiring other electrical connections.
Another way of tapping a SAW delay line is piezoelectric leaky wave coupling across an air gap. Heretofore, this has only been used to couple surface waves from one surface to another surface; each surface being on a substrate of the same type of material.
Since identical materials have identical velocities of propagation, the coupled SAW has the same delay time as a SAW which traveled an equal path lenth on the original substrate. In this case, there is a single input and a single output. Because the original SAW is not preserved, this structure is not utilized as a tapped delay line but as a means of transferring SAW energy from one substrate to another. In the applicant's invention, piezoelectric leaky wave coupling across an air gap is utilized to construct a tapped delay line which preserves the original SAW and provides additional, multiple, delayed and timeresolved pulse.
A further method, disclosed by Wang in U.S. Pat. No. 3,684,970, utilizes a thin fluid layer between a piezoelectric crystal and a piezoelectric semiconductor place adjacent to each other. This form of coupling utilizes acoustical coupling obtained by means of the fluid layer being in contact with both surfaces to transfer the elastic wave from one surface to the other surface. The disadvantages of this technique are the mass loading of both surfaces which introduces loss, and the results in the initial surface wave pulse being strongly attenuated. Furthermore, the use of a liquid layer interface has disadvantages in fabrication, lifetime of the device, and reproducibility. Wang's device utilizes elastic coupling whereupon the vibrations of the particles in the substrate cause the particles in the liquid to vibrate transferring energy to the liquid, since both the liquid and the substrate are in contact with each other. The vibration of the particles in the liquid then causes the particles in the semiconductor substrate to vibrate since their surfaces are also in contact with each other. In other words, the liquid interface acts to combine the elastically the substrate and semiconductor material together. Applicant's device, on the other hand, does not combine the materials together to form a composite elastic structure. An SiO.sub.2 spacer is used to prevent mass loading, but the spacer is located outside of the surface area where the surface acoustic wave propagates; this means that in the region where the surface wave propagates, the substrate material and the semiconductor material are not elastically coupled.
Applicant's device also utilizes a bulk wave amplifier in the piezoelectric semiconductor to amplify the bulk wave by the piezoelectric leaky wave coupling across the air gap. The semiconductor medium is electrically biased so that charge carriers drift with a velocity component parallel to the preferred propagating direction of bulk acoustic waves traveling in the semiconductor medium. Since the semiconductor material is piezoelectric, there is associated with the acoustic wave an electric field which can interact with the charge carriers in the semiconductor. In analogy to the electron-field interaction in a traveling wave tube, charge carriers radiate energy into the acoustic media if their velocity is greater than that of the acoustic wave. This, in turn, will cause the acoustic waves to grow in amplitude, and like the traveling wave tube, the growth is exponential versus distance. If the charge carriers travel slower than the acoustic wave, then the charge carriers absorb energy from the wave and the device acts as an attenuator. It should be noted that in the traveling wave tube, the charge carriers undergo no collisions, while in the ultrasonic amplifier, the mean free path of the charge carrier is short compared to the acoustic wavelength.
This type of amplification was first reported in a paper by A. R. Hutson, J. H. McFee, and D. L. White, entitled "Ultrasonic Amplification in CdS," appearing in Physical Review Letters, Vol. 7, September 1961, pp 237-239. There, 15 and 45 MHz acoustic waves were found to change in amplitude with voltage applied to the piezoelectric semiconductor, cadmium sulfide. When the electric field was such that the drift velocity of the electrons exceeded the velocity of sound, the acoustic waves were amplified as they traveled through the crystal. In this initial case of amplification, the acoustic waves were generated in the CdS by bulk wave transducers. In Wang's patent, mentioned previously, the bulk wave in the CdS was generated by elastically coupling a surface acoustic wave across a liquid layer to the piezoelectric media. In the applicant's device, the bulk wave generated in the CdS is obtained by piezoelectrically coupling a surface acoustic wave across an air gap to the CdS. In all cases, the amplification technique is the same, only the structures for coupling or generation of the bulk waves is different.